Anxiety and fear are emotions that are vital for individuals when dealing with difficult challenges and threats. However, persistent vigilance and frequent recall of memories associated with a fearful experience can lead to generalized anxiety disorder, posttraumatic stress disorder (PTSD), and phobias. Orexins (hypocretins) are neuropeptides that play a critical role in promoting arousal. Accumulating evidence suggests that orexin neurons are activated in animals exposed to stressful conditions and that orexins are involved in the regulation of anxiety and fear. In this thesis, the role of the orexins in anxiety and fear was studied using behavioral, biochemical, and pharmacological methods. Experiments were done to identify some of the possible mechanisms involved. First, microinjections of orexin peptides (orexin-A and orexin-B) into the paraventricular nucleus of the thalamus (PVT) induced anxiety-like behaviors. The anxiogenic effect produced by microinjections of orexin-A in the PVT was attenuated by blocking receptors for the stress-related peptides, corticotropin-releasing factor (CRF) and dynorphin. Second, the mRNA levels for prepro-orexin (ppOX) and the orexin 1 receptor (OX1R) were found to be increased in the posterior hypothalamus of shocked rats at 2 weeks post-shock. Third, 2 weeks after the footshocks, an enhanced level of CRF mRNA in the amygdala and ppOX mRNA in the posterior hypothalamus were detected in a subset of shocked rats that displayed an acute fear response to a novel tone and that eventually resulted in a generalized anxiety (high responders, HR). Fourth, the dual orexin antagonist TCS1102 (20 mg/kg; i.p.) and OX1R antagonist SB334867 (20 and 30 mg/kg; i.p.) decreased fear level when tested at 2 weeks post-shock. These results suggest that endogenously released orexins regulate anxiety through acting on the PVT and that the brain kappa and CRF systems could be involved. The results also indicate that changes in orexins and CRF occur specifically in HR. These findings provide a better understanding of the brain mechanisms contributing to hyperarousal and anxiety. In addition, the results support the use of orexin receptor antagonists, especially OX1R antagonists, in the treatment of anxiety disorders including PTSD, panic attacks, and phobias.